Composite metal and concrete deck floor systems are known. These composite structures include a metal deck and a concrete slab supported on the metal deck. More particularly, one type of composite deck system comprises a concrete slab which is reinforced and supported by a cold formed steel deck. These two basic components work together to provide superior load carrying capability. The metal deck material is of a uniform thickness. In some traditional designs, the cold formed metal deck is shaped such that there is provided a generally horizontal bottom with portions of the deck bent and shaped so as to project upwardly from the bottom. The metal deck material is of a uniform thickness. The horizontal portion of the metal deck serves as a form for the concrete. In addition, the horizontal portion of the metal deck functions as a positive reinforcement for the structural concrete slab. The upstanding portions of the metal deck that project upwardly from the bottom also adds load carrying capacity to the deck assembly.
There are generally three phases to the construction of such a deck. First the metal decking is laid down over supports. Sometimes intermediate and temporary supports may be required. The decking must support itself and also the personnel and equipment needed to install it. As noted above, in some conventional metal decks for flooring, there is provided upstanding structures formed in the metal itself. The tops of these upstanding structures are generally in compression while the horizontal or bottom portions of the deck are in tension. It is well recognized that steel is most efficient when in tension. Next, concrete is poured onto the metal deck. There is no composite action yet, so the metal deck must now support the added weight of the concrete. In the third phase, the concrete hardens and the composite action takes place between the metal decking and the concrete. The top of the concrete slab is in compression which, as is well known, is where concrete is most efficient. As one looks down through the concrete, more tension forces are experienced and the further down in the concrete, the weaker the concrete becomes. The steel portion or the metal deck portion now comes into play and provides additional tensile strength to the assembly.
There are several limitations or disadvantages to such a conventional composite metal-concrete deck floor. First there is a tradeoff in decking metal thickness. A heavier gauge metal is unnecessary in the bottom or horizontal portion of the metal deck but would add load carrying capacity to the assembly in the structural portions of the deck that project upwardly from the horizontal or bottom portions, especially where the steel is in compression. Conversely, a lighter gauge metal would save on material in the horizontal portion of the metal deck. But this would reduce the load capacity where the steel is in compression.
In addition when a load is applied to a composite metal-concrete deck in a horizontal direction, as may be experienced in a progressive collapse condition or extreme loading event, the decking is subject to spreading apart in the horizontal direction. This could result in a failure of the entire floor system. This is due to the poor tensile strength of concrete and the fact that the metal deck typically does not provide reinforcement that resists such loads.